1. Field of the Invention
The present invention relates to a toner to be used in image formation based on electrophotography such as copiers, printers, facsimiles, and electrostatic recording devices. The present invention also relates to an image forming apparatus, an image forming method and a process cartridge each of which uses the toner.
2. Description of the Related Art
For image forming apparatuses such as printers, copiers and facsimiles, in recent years, market demands for energy saving and increases in speed have increased. In order to achieve the performance of these demands, it is important to achieve improvements in thermal efficiency of a fixing unit used in an image forming apparatus.
Generally, in an image forming apparatus, an unfixed toner image is formed on a recording medium such as recording sheet, printing paper, photosensitive paper and electrostatic recording paper by using a direct transfer method or an indirect transfer method through an image formation process such as electrophotographic recording, electrostatic recording and magnetic recording. As a fixing unit used for fixing such an unfixed toner image, for example, contact heating methods, such as heating roller method, film heating method and electromagnetic induction heating method, are widely employed.
Further, for an electrophotographic toner (hereinafter, may be simply referred to as “toner”), with higher speed performance and energy saving of an image forming apparatus, a toner which is excellent in low-temperature fixing property is requested. In the meanwhile, there is a need for a toner having properties of anti-offset property and storage stability (blocking resistance) which are contradictory to the low-temperature fixing property.
To meet the needs, a variety of toners using aromatic polyester resins are proposed, but they have a disadvantage in that they are poor in pulverizability in their production process. Then, to address the problem, a method is proposed in which a low-molecular weight polyester resin prepared using an aliphatic alcohol that is excellent in pulverizability as a monomer is blended with a highly polymeric polyester (see Japanese Patent Application Laid-Open (JP-A) No. 2002-287427). However, when the low-molecular weight polyester resin prepared using an aliphatic alcohol is used, the storage stability of the toner is degraded because the polyester resin has a low glass transition temperature due to its structure and it is difficult to satisfy low-temperature fixing property, anti-offset property and storage stability at a high level. In addition, since the low-molecular weight polyester resin prepared using an aliphatic alcohol has low mechanical strength, it may cause a problem with occurrence of toner fine particles and contamination inside the used developing device and developing sleeves when the toner is agitated and sheared in the developing device.
Further, in a fixing unit used in an electrophotographic image forming apparatus, the used toner is requested to have releasing property (may be referred to as “anti-offset property”) to heating members is required. The anti-offset property can be improved by containing a releasing agent in a toner, and how the releasing agent is present inside the toner greatly affect the releasing property. It is desired that the releasing agent be uniformly dispersed in the toner in terms of stability of the quality, however, when the releasing agent is excessively finely dispersed in the toner, the releasing agent hardly exudes out of the toner surface at the time of heating. Thus, it is preferable that the releasing agent be dispersed so as to have a moderate dispersion diameter. However, for a polyester resin prepared using an aliphatic alcohol, there is a problem that it is highly soluble in generally used releasing agents and is poor in releasing property.
Further, methods are proposed in which a resin grafted with a vinyl resin is used as a dispersing agent in a releasing agent such as polyolefin, and the releasing agent is moderately dispersed in a toner (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 2000-305319, 2003-98726, Japanese Patent (JP-B) Nos. 3597525 and 3801487). However, dispersing agents prepared in accordance with these proposals are not suitable for polyester resins prepared using aliphatic alcohol, and these proposals are incapable of improving the releasing property of toner.
Therefore, as for a toner that is excellent in all of the low-temperature fixing property, anti-offset property, storage stability, mechanical strength, anti-fouling property to developing devices, toner filming resistance to latent electrostatic image bearing members and carrier particles, and pulverizability and is capable of forming high-quality images over a long period of time, and as for an image forming apparatus and related technologies which are capable of forming extremely high-quality images using a toner with no change in color tone being recognized and with no abnormal images such as a reduction in image density and background smear being observed for a long period of time, those having satisfactory performance have not yet been provided so far. Currently, further improvements and developments are desired.